Redshift

[touqra]

If light experiences gravitational redshift as said by GR, then, where did the difference between the initial and final energy, E = \frac {hc}{\lambda} go?

From Hubble expansion, there is also redshift. Where did the energy difference go?

[asdfjkl]

The gravitational redshift is caused by the photon escaping a gravitational potential well. It is just like a ball being thrown up into the air faster than the escape velocity. Eventually, the ball will escape the earth, but it will be moving slower than when it was thrown (it lost some kinetic energy). Since light is affected by gravity, it can have gravitational potential energy just as a massive ball does.